(1) Field of the Invention
The invention relates to the use of a double destruction phase shift mask and to methods of forming the double destruction phase shift mask. The double destruction provides improved pattern definition and depth of focus in the sub-half micron range and avoids the problem of ghost lines which can occur with conventional phase shift masks.
(2) Description of the Related Art
As optical lithography advances to 0.5 to 0.35 microns and below new technologies are needed to improve the resolution of the imaging lens. Phase-shifting photomasks have been used to improve resolution as well as depth of focus. Phase shifting processes are described in U.S. Pat. No. 5,045,417 to Okamoto, U.S. Pat. No. 5,194,344 to Cathey Jr. et al, U.S. Pat. No. 5,194,345 to Rolfson, U.S. Pat. No. 5,194,346 to Rolfson et al, U.S. Pat. No. 5,208,125 to Lowrey et al, U.S. Pat. No. 5,217,830 to Lowrey, and U.S. Pat. No. 5,225,035 to Rolfson. The methods used in these phase shifters usually use an opaque material in conjunction with a phase shifting material. U.S. Pat. No. 5,190,836 to Nakagawa et al, describes a phase shifter using reflection techniques. Phase shift techniques are described in a paper "LITHOGRAPHY'S LEADING EDGE, PART 1: PHASE-SHIFT TECHNOLOGY," published in Semiconductor International, February 1992, pages 42-47.